This invention relates to an on-line linear tonometer and more particularly to apparatus and method for providing accurately tonometered aqueous buffer solutions and whole blood on a continuous basis for use in the quality control of blood gas analyzers.
The measurement of pH and the partial pressures of oxygen (pO.sub.2) and carbon dioxide (pCO.sub.2) in blood samples is routinely performed in hospitals, laboratories and other institutions because of the significance of such information in relation to a patient's cardiovascular, respiratory and metabolic status.
These measurements are carried out by blood gas analyzers which may be manual, semiautomatic, or completely automatic or automated in operation. The fully automatic units are becoming increasingly popular because of their ability to conduct measurements rapidly, typically up to twenty samples an hour, and accurately, provided calibration is properly maintained.
Checking of calibration, that is, quality control, in an automated blood analyzer can be expensive and inconvenient. For example, the most popular type of automated blood gas analyzer in use today relies on three electrode systems to determine levels of pH, pCO.sub.2, and pO.sub.2. There is a separate electrode system for each of the above factors. As the blood sample is exposed in turn to each electrode an electrical output is produced which is proportional to the factor to which that electrode is sensitive.
Calibration of these olood gas analyzers involves adjusting the interpretation of actual electrode output based on the measurement of samples with known levels of pH, pCO.sub.2, and pO.sub.2. Such samples are provided by the manufacturer in the form of pH buffers and calibrated gas. These substances are inserted into the analyzers at intervals specified by the manufacturer with the readout of the machine adjusted accordingly to reflect the known values. Typically, a manufacturer might indicate intervals of two hours of use for a two-point calibration (adjusting the electrode response at two levels) and every thirty minutes of operation for a onepoint calibration (electrode response at one level).
The performance of blood gas analyzers, which are capable of making very precise measurements, frequently is subject to degradation of accuracy due to a variety of factors such as: coating of the electrodes with blood proteins, loss of internal electrolyte from electrodes, plugging of internal pathways by minute blood clots, etc. It is essential therefore to repeatedly analyze samples of known concentration throughout the active work period. The analysis of known samples is termed quality control testing. This invention is a device for producing such a quality control product for use in such testing.
With regard to the tonometered samples employed to quality control the blood gas analyzer, it is obvious that tonometered whole blood would be preferred, but blood s relative instability, its unreliability as a pH reference, procurement difficulties, and biohazard potential make it a less than acceptable fluid for general use. As a consequence, there have been developed certain liquids capable of holding CO.sub.2 and O.sub.2 in solution in known amounts, and a variety of such solutions are available packaged in one-time-use vials or ampules.
The basic techniques involved in preparing tonometered buffered solutions or whole blood are described and illustrated in a journal article entitled "Quality Control in Blood pH and Gas Analysis by Use of a Tonometered Bicarbonate Solution and Duplicate Blood Analysis" appearing in Clinical Chemistry, Vol. 27, No. 10, 1981, pages 1761-1764. A prepared mixture of CO.sub.2, O.sub.2, and N.sub.2 is bubbled up through the buffered solution until equilibration occurs at which time the solution is ready for use. As noted in the article, there are severe problems associated with the described arrangements.
An institution employing the blood gas analyzer can either prepare its own tonometered liquid samples for quality controlling its machine, or, as is more commonly the case, purchase the tonometered samples packaged in one-time use ampules as noted above. Because of the costs involved and other problems, hospitals, laboratories, and other institutions employing blood gas analyzers generally do not prepare their own samples for quality control. Equipment to prepare large amounts of tonometered solutions and package them in single use ampules for storage is prohibitive in cost and not practical for an institution not having a factory environment.
However, the cost of utilizing purchased ampules containing tonometered liquid samples for quality control is also quite high. In a typical hospital with an automated blood gas analyzer requiring nine calibrations a day, the cost of the ampules containing the tonometered solutions runs about two dollars each or between six and over seven thousand dollars per year. If more than nine assays a day are required, the cost will run even higher. Where there is present more than one analyzer it can be seen that the costs to the institution of maintaining its blood gas analyzers can be a substantial factor in providing medical services.
Another drawback in the use of ampules has to do with their questionable accuracy, which is in the range of 5-10%, probably due to the effects of packaging, shipping, and storage. Ampules, however, are currently the method of choice for quality control due to the lack of a better alternative.
In U.S. Pat. No. 4,358,424 to Weber et al which is directed to the tonometering of blood plasma, there is shown online tonometering apparatus which is excessively complex and expensive in construction due to the built-in frit construction, the double syringes for each channel, and the relatively complicated gas supply configuration. In addition the use of open vessels creates potential contamination. It appears also that Weber et al contemplates intermittent operation of the device since apparently a 40-minute waiting period is specified for equilibration so that it is not available at all times for use.